Stay on top of your fluid game

Even a slight level of dehydration, just one percent (1.5 lbs for a 150-pound athlete), can contribute to a five-percent decline in metabolic efficiency.
Determining fluid and electrolyte needs is perhaps the most important step in helping optimize performance, as dehydration's effects are widespread and serious. To really understand the importance of hydration, let's explore water's role in human performance.

1. Water is the medium for metabolic activity.
In order to properly metabolize the calories ingested during activity, an athlete needs to be hydrated. Even a slight level of dehydration, just one percent (1.5 lbs for a 150-pound athlete), can contribute to a five-percent decline in metabolic efficiency.

What does this mean for the athlete? First off, the calories being ingested, especially solid calories, will be left in the stomach rather than being distributed to the working muscles, leading to premature muscle fatigue.

Any fluids ingested will collect in the belly until there's a proper concentration for digestion. This leads to a shortage of fluid being directed to working muscles, and results in muscle cramping. During a high-impact activity, all the food and fluids left in the belly will be jiggling around, leading to uncomfortable side stitches, nausea, vomiting and diarrhea.

2. Water is a lubricant for our muscles and joints.
Noticeable muscle aches/pains/cramps can occur at the slightest bit of dehydration and become debilitating as dehydration becomes more severe. Headaches are also a common complaint.

3. Water helps cool the body
Water can be compared to the coolant we use in our cars. When the coolant runs low, our cars overheat. Our bodies perspire in order to lower our internal temperature; as blood flow to the skin increases the internal heat generated evaporates through sweat. An athlete may have a flushed or blotchy appearance.

As dehydration becomes more severe, this process becomes compromised causing core body temperature to elevate. The athlete may get the chills or goose bumps when the heat being generated isn't released from the body efficiently. If left untreated, dehydration can lead to heat exhaustion or heat stroke and potentially death.

For every percent drop in hydration, expect a three- to five-percent decline in performance. This is huge when you think about it -- it would be like adding an additional one to two minutes onto a 40-minute 10k runner's time. Thirst isn't a good indication of when an athlete needs to start drinking, as our thirst mechanism is initiated upon a three-percent level of dehydration which equates to a 10- to 15-percent drop in metabolic efficiency, cooling efficiency, muscle function and overall performance.

Determining sweat rate

Any athlete preparing themselves for peak performance should be on top of their fluid game -- they need to know their individual sweat rate and consequent fluid and electrolyte replacement needs. To determine sweat rate, weigh yourself both immediately pre- and post-exercise on several different occasions making note of environmental conditions and the intensity of the workout.

Every pound of body weight lost during exercise is equivalent to approximately 16 ounces of fluid. For example, if you consistently lose one pound on a 30-minute run in which no fluids are consumed, hourly fluid needs equal 32 ounces per hour, which is actually the upper end of norm for most athletes.

Since 1988, Gatorade Sports Science Institute (GSSI) Laboratories in Barrington, Illinois, have been helping athletes determine sweat rate, providing valuable scientific research and education in the areas of exercise, sports science and nutrition.

In August 2004, I had the opportunity to visit the GSSI lab to undergo a sweat test, which entailed measuring fluid loss and sweat electrolyte content during one hour of moderate-intensity exercise in warm conditions in their labs. Prior to starting the test, my weight was taken and recorded. I was asked to set the speed at an intensity that I was able to maintain easily for an hour. I was able to freely consume Propel Fitness Water during the run.

During the test, sweat patches were strategically placed on several areas of my body to help measure the electrolyte composition in my sweat. After an hour of running, I toweled off the sweat and once again took my weight.

Results of my sweat test at GSSI are shown on the table below and compared to pro triathlete Chris Leigh, who also was recently measured at GSSI Labs. With a fluid intake of only eight ounces and a total weight loss of just under two pounds, my calculated sweat rate was measured at 39 ounces per hour, as compared to the 74 ounces per hour lost by Chris.

The sweat patches were taken off and run through a series of lab tests which helped determine the salt concentration in my sweat, which was 172 milligrams per eight ounces, a concentration far below the Chris Leigh, who has a salt concentration of 350 milligrams per eight ounces.

Kim Mueller Chris Leigh

Athlete Description 28-year-old competitive female age-group triathlete and runner with no history of muscle cramping. 31-year-old Australian pro triathlete, who is a two-time Ironman Triathlon champion, winning the 2000 Ironman California and the 2004 Ironman Coeur d'Alene. In 1997 during the Hawaiian Ironman, Chris experienced the extremes of dehydration, including vomiting, diarrhea and severe stomach cramps that led him to collapse just 50 meters from the finish. Shortly after, he had surgery to remove one third of his large bowel, as it had died as a result of dehydration (his body stopped supplying oxygen and nutrients to his large bowel so that blood could continue flowing to his heart, lungs and muscles).

Exercise Protocol 1 hr treadmill running at 9.1 mph in a chamber with a constant 76 F temperature and 70% humidity. 4 hrs of stationary cycling and treadmill running in 88 F and 71% humidity.

Total Fluid Loss 1.15 liters 8.8 liters

Hourly Fluid Needs 39 fluid ounces 74 fluid ounces

Total Salt Loss 840 mg 12,953 mg

Hourly Salt Needs 840 mg 3,238 mg

Take-home message

To ensure optimal absorption of calories, peak muscle function and efficient cooling of the body, it's essential that athletes stay on top of their fluid game. Determination of sweat rate, which can be calculated by evaluating total fluid intake and weight loss during activity, should be a high priority for athletes looking to maximize performance and protect against serious injury and/or health consequences such as heat stroke.

Electrolytes need to be added to fluids when training in heat and/or training duration extends beyond an hour. Most athletes require .5 to 1 liter of fluid per hour along with .5 to 1 gram of sodium during prolonged training to prevent the detriments associated with fluid and/or electrolyte imbalances.

Kimberly J. Mueller, M.S., R.D., is a registered sports dietitian and competitive endurance athlete who provides nutritional counseling and meal planning to athletes worldwide. For more information about Kim, visit or contact her at

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